The blue egg sac is an item obtained from looting the Grubby Chest in the south-west corner of the Forthos Dungeon. It is used on Sraracha to allow it to metamorphose into the magic-attacking Spawn of Sarachnis.
Upon using it on Sraracha, the chatbox message states In a scene so disgusting the artists refused to animate it, Sraracha brutally devours the egg sac. It can now turn blue on command. This is one of few occasions where the fourth wall is broken within the game.
For an exhaustive list of all known sources for this item, see here